Intervertebral discs are soft and compressible. They are interposed between adjacent vertebral body elements of the spine. They act as shock absorbers for the spine, allowing it to flex, bend, and rotate. Degenerative disc disease can occur throughout the spine, but most often occurs in the discs in the lower back (lumbar region) and the neck (cervical region).
As the process of degeneration continues, micro tears or cracks occur in the outer layer (annulus fibrosus) of the disc. The jellylike material inside the disc (nucleus pulposus) may be forced out through the tears or cracks in the annulus, which causes the disc to bulge, break open (rupture), or break into fragments.
The economic impact of degenerative disc disease is enormous accounting for a significant morbidity and lost wages.
The physical properties of the disc are the nucleus pulposus which is composed of type II collagen and the annulus fibrosis which surrounds the disc and gives it significant form. The annulus composed of type I collagen. The nucleus pulposus is largely made up of molecules called proteoglycans. These proteoglycans have an affinity for water. It is this retention of water and the stoichiometry of folded molecules that is responsible for the unique mechanical properties of the disc. If these proteoglycans are depleted, the discs become more rigid and the loss of fluid results in a disc that is thinner and less compliant. Clinically this results in narrowing of the distances between the vertebral elements. This is best seen on magnetic resonance imaging. Typically discs have a bright signal on T2 pulse-weighted sequences and they are hypointense on corresponding T1 images. This is due to the high fluid content of the discs. As the disc loses fluid i.e. the loss of proteoglycans, the disc loses its water signal and becomes anhidrotic and eventually mineralizes. As a result, these individuals develop the symptoms in the spine contributable to loss of the normal disc architecture. As the process of degeneration continues, one develops micro tears or cracks and fissures in the annulus fibrosis and through these cracks and fissures the nucleus pulposus, which is largely gelatinous, may extrude. The extruded disc material may efface the dura and cause significant nerve compression which may result in traumatic neuritic pain and or motor loss. Therefore, once these early changes in disc degeneration are recognized, it may be prudent to replenish the disc with proteoglycans. Currently, synthetic and artificial substitutes are used to stimulate repair.